This invention relates to devices for integrated optics and, more particularly, to thin-film optical devices employing a grating structure as a lens.
Considerable effort has been devoted to improving the quality of guided-wave optical lenses. The following references disclose much of this effort. S. K. Yao and D. B. Anderson, "Shadow Sputtered Diffraction-Limited Waveguide Luneburg Lenses", Appl. Phys. Lett. 33, 307-309, (1978). W. H. Southwell, "Geodesic Optical Waveguide Lens Analysis", J. Opt. Soc. Am. 67, 1293-1299 (1977). V. E. Wood, "Effects of Edge-Rounding on Geodesic Lenses", Appl. Opt. 15, 2817-2820 (1976). C. M. Verber, D. W. Vahey, and V. E. Wood, "Focal Properties of Geodesic Waveguide Lenses", Appl. Phys. Lett. 28, 514-516 (1976). E. Spiller and J. S. Harper, "High Resolution Lenses for Optical Waveguides", Appl. Opt. 13, 2105-2108 (1974). D. B. Anderson, R. L. Davis, J. T. Boyd and R. R. August, "Comparison of Optical Waveguide Lens Technologies", IEEE J. Quantum Electron., QE-13, 275-282 (1977). D. B. Anderson and R. R. August, "Progress in Waveguide Lenses for Integrated Optics", Trans. IECE Japan E61, 140-143 (1978). D. Kassai, B. Chen, E. Marom, O. G. Ramar, and M. K. Barnoski, "Aberration Corrected Geodesic Lens for Integrated Optics Circuits", Topical Meeting on Integrated and Guided Wave Optics, Tech. Dig., Salt Lake City, 1978. P. K. Tien, "Methods of Forming Novel Curved-Line Gratings and Their Use as Reflectors in Integrated Optics", Opt. Lett. 1, 64-66 (1977). P. R. Ashley and W. S. C. Chang, "Fresnel Lens in Thin Film Waveguides", Topical Meeting on Integrated and Guided Wave Optics, Tech. Dig., Salt Lake City, 1978. As a result of this work, diffraction-limited guided-wave optical lenses have been constructed using a semiplanar technology for thin-film Luneburg lenses. In this connection, refer particularly to the work of Yao et al listed above. Bulk optics technology has been used to construct geodesic lenses. However, the topical alignment of both Luneburg lenses and geodesic lenses is difficult because the alignment procedure requires the positioning of tools. Another barrier to the use of Luneburg and geodesic lenses arises because their focusing properties vary as a result of variations in the process by which they are fabricated. As another disadvantage of the Luneburg lens, they require the use of a film having a higher index of refraction than that of the waveguide material. Thus, Luneburg lenses are limited to applications where the waveguide material has a relatively low index of refraction. Another disadvantage of the geodesic lenses is that they require aberration correction.
Some of the difficulties and disadvantages of Luneburg and geodesic lenses have been overcome by employing photolithographic and electron-lithographic techniques to construct a new family of planar grating lenses for guided-wave optics. The new family of lenses includes curved grating reflectors, Fresnel zone phase plates, and Fresnel Bragg deflector lenses. Curved grating reflectors are discussed in the paper by P. K. Tien listed above. Fresnel zone phase plates are discussed in the paper by P. R. Ashley et al listed above. Fresnel Bragg deflector lenses are discussed by G. I. Hatakoshi and S. I. Tanaka in "Grating Lenses for Integrated Optics", Optics Letters, June 1978, Volume 2, No. 6, pp. 142-144. The grating lens disclosed by Hatakoshi et al has a non-uniform periodicity which varies parabolically in a symmetrical fashion about the center of the grating. The grating lines or elements are all rotated or tilted with respect to each other. The latter feature has the disadvantage of making the lens difficult to manufacture by digital computer techniques. In addition, the focus of the grating lens of Hatakashi et al is positioned in the through beam of the grating. The through beam of a grating is the zero order beam transmitted through the grating in line with the incident beam. This feature has the disadvantage of focusing the information power at a point where the noise power is relatively high.
Some of the characteristics which are desired for lenses are high throughput, low spurious noise, i.e., low side lobes, and diffraction-limited performance. Thus far, experimental results obtained using the lithographically produced types of lenses listed above have been disappointing. Good quality curved grating reflectors are difficult to make because of the small radius of the grating lines. Scanning electron beams and other digitally controlled pattern generators are awkward to use in making the curved grating lines. Good focusing properties have been obtained in Fresnel zone phase plate lenses. However, the Fresnel lens inherently has many focal points, all of them lying along the optical axis. Thus, the other focal orders constitute a noise background for the desired focal order. The performance results published thus far for the Fresnel Bragg deflector lens are not particularly good. This is probably due to the difficulties encountered in generating rotated or tilted grating elements and other experimental imperfections. In addition, for this lens also, the focal points all lie along the optical axis increasing the noise background for the desired focal order.
Acousto-optic Bragg beam deflectors, wherein the waveform of a bulk acoustic wave results from a linear chirp, i.e., a linear variation in frequency, have been used to focus the energy of optical beams to a moving focal spot. Acousto-optic beam deflectors and their focusing effect are discussed in the following references: E. I. Gordon, "A Review of Acoustooptical Deflection and Modulation Devices", Proc. IEEE, 54, p. 1391, 1966; J. R. Boyd, E. H. Young, and S. K. Yao, "Design Procedure for Wide Bandwidth Acousto-Optic Modulator", Opt. Eng. 16, 452-454 (1977); S. K. Yao and E. H. Young, "Acousto-Optical Multiplexer for Fiber Optical Systems", IEEE Proc. of Ultrasonics Symposium, Annapolis, Md., 1976, pp 214-217; and L. D. Dickson, "Optical Considerations for an Acoustooptic Deflector", Appl. Opt. 11, 2196-2202 (1972). Of course, the movement of the focal spot produced by acousto-optic beam deflectors makes these devices impractical for those integrated optics applications which require a fixed or stationary lens.